This mode should be much faster than the fallback Pixmap based rendering as it is GPU accelerated and the Colourspace Conversion step can be skipped as it supports YUV pixel formats natively.

Notes

Some drivers are blacklisted for opengl support, for example: older versions of the nouveau driver on Linux, and the Intel driver on all platforms.

Status messages should be printed out when using the command line tool to attach to a session (unless the configuration file or command line options specifically disabled it).
The --opengl=yes|no|auto option can be used to override the automatic detection of supported drivers.

You can also find the opengl driver information in the Features pane of the Session Info dialog from the system tray menu.